Recent semiconductor devices includes a couple of polysilicon plugs, which are superimposed in a vertical or thickness direction. Such superimposed polysilicon plugs are electrically insulated by an insulating material. According to a conventional semiconductor device, a material having a high flow rate (flow characteristic) is generally employed, such as BPSG, for the insulating material, to provide a good flatness. However, impurity, such as boron, may be diffused from the insulating material into the polysilicon plugs. As a result, the electrical resistance of the poly-silicon plugs may be increased.